Glucose is the primary source of fuel for all body cells. After a meal, some of the glucose not used immediately for fuel travels to the liver or skeletal muscles, where it is converted to glycogen through the process glycogenesis, and stored for energy. Any excess glucose is stored in adipose tissue as fat. The liver has a greater capacity for glycogen storage than the muscles. Liver cells can typically store up to 8% of their weight as glycogen, while muscle cells can typically store up to only 3%. The liver is responsible for maintaining adequate levels of glucose in the body. As the body's glucose level drops, the liver converts some of the glycogen back into glucose through the process glycogenolysis and releases it back into the bloodstream. Muscle cells, on the other hand, are unable to reconvert glycogen to glucose. Instead, they convert glycogen directly to fuel through the process glycolysis.
Glycolysis is the cellular anaerobic process, which breaks down muscle glycogen into pyruvic acid during high-intensity exercise. This process rapidly produces a small amount of adenosine triphosphate (ATP), the necessary fuel for body cells. However, if too much pyruvic acid accumulates in the muscle during glycolysis, it can substantially slow down or even stop the process of ATP formation. Therefore, after one or two minutes of high-intensity exercise, a subsequent process of energy formation begins. This process is referred to as oxidation.
Oxidation produces over 95% of the energy used by muscles during moderate and prolonged exercise. Oxidation immediately converts much of the pyruvic acid formed through glycolysis to ATP. However, during prolonged exercise, if an athlete is unable to breathe in oxygen quickly enough to oxidize pyruvic acid into ATP, some pyruvic acid is converted to lactic acid and diffused out of the cell. It then circulates throughout the body until it can be reconverted to pyruvic acid once oxygen again becomes available. If excess accumulation of lactic acid occurs, extreme fatigue can set in, which can greatly impair the athlete's performance.
Glucose is needed by the central nervous system to keep the body functioning. Therefore, during periods of moderate exercise lasting longer than 20 minutes, the body works to conserve stored muscle and liver glycogen. It does so by reducing the percentage of fuel derived from glycogen to only 40% or 50%, with the remainder supplied by fat. During exercise periods lasting longer than 4 or 5 hours, as much as 60% to 85% of fuel produced by oxidation may be derived from fat.
Fats need carbohydrates in order to burn efficiently. The breakdown of carbohydrates generates oxaloacetic acid, which is needed for the breakdown of fats into fuel. If insufficient carbohydrate levels exist, the levels of oxaloacetic acid may also drop, making it difficult for the body to continue producing a high level of fuel from fat. Although the body can break down fats in the absence of carbohydrates, it does so at a much slower rate. When the glycogen stores in the muscles and liver are depleted, and the blood glucose level begins to fall, athletes begin to experience fatigue, lack of coordination, light-headedness and lack of concentration. This experience is commonly known as “hitting the wall” or “bonking”.
During exhaustive exercise the aim is to increase sparing of muscle glycogen and thereby simultaneously extend endurance. There is a consensus that 8 to 10 g of carbohydrate per kg of body weight will maintain appropriate glycogen levels during heavy training.
Following exhaustive exercise, the body needs to replenish the depleted glycogen reserves. Furthermore, the muscle degradation during exercise requires protein to fully recover. It is therefore important to consume additional carbohydrate and protein after exercise. This should be done within the first two hours after exercise during the period known as “the muscle recovery window” or “the glycogen replacement window”. This is because the enzyme glycogen synthase, which is responsible for restoring glycogen, is highly elevated immediately after exercise. A combination of carbohydrate and protein is recommended preferably together with water and electrolytes.
When the body has sustained a complete or a bear-total depletion of its glycogen stores, it will take approximately 24 hours for the body to both ingest sufficient food of the appropriate carbohydrate proportion as well as convert the ingested carbohydrates into glycogen.
The pattern of muscle glycogen synthesis following its depletion by exercise is biphasic. Initially, there is a rapid, insulin independent increase in the muscle glycogen stores. This is then followed by a slower insulin dependent rate of synthesis. Contributing to the rapid phase of glycogen synthesis is an increase in muscle cell membrane permeability to glucose, which serves to increase the intracellular concentration of glucose-6-phosphate (G6P) and activate glycogen synthase. Stimulation of glucose transport by muscle contraction as well as insulin is largely mediated by translocation of the glucose transporter isoform GLUT4 from intracellular sites to the plasma membrane. Thus, the increase in membrane permeability to glucose following exercise most likely reflects an increase in GLUT4 protein associated with the plasma membrane. This insulin-like effect on muscle glucose transport induced by muscle contraction, however, reverses rapidly after exercise is stopped. As this direct effect on transport is lost, it is replaced by a marked increase in the sensitivity of muscle glucose transport and glycogen synthesis to insulin. Thus, the second phase of glycogen synthesis appears to be related to an increased muscle insulin sensitivity. Although the cellular modifications responsible for the increase in insulin sensitivity are unknown, it apparently helps maintain an increased number of GLUT4 transporters associated with the plasma membrane once the contraction-stimulated effect on translocation has reversed. It is also possible that an increase in GLUT4 protein expression plays a role during the insulin dependent phase.
Muscle glycogen is an essential source of energy during endurance training and competition (Vandenbogaerde et al. 2011). Several studies have shown that depletion of muscle glycogen storages coincides with fatigue during prolonged physical activity (Hermansen et al. 1967, Coyle et al. 1986). Further it has been shown by Costill et al. that it is not possible to maintain a high aerobic power output after the muscle glycogen storages has been depleted (Costill et al. 1971). Thus, glycogen depletion impairs the performance in endurance sports disciplines such as long distance running, cycling, triathlon, cross-country skiing etc.
In line with this, it has been found that the ability to maintain optimal aerobic performance during long lasting endurance activities is directly dependent on the initial size of the muscles glycogen storage (Jeffrey et al. 1993). Furthermore, it has also been shown that a single exhaustive training session could be sufficient to deplete the glycogen storages to a degree which would have a negative impact on performance in a following session.
At a glycogen storage rate of 5-6 mmol/kg/ww/h it may take up to 24 hours, to replenish the glycogen storage after exhaustive exercise (Coyle et al. 1986), this is supported by results from numerous other studies (Blom et al. 1987; Ivy et al. 1988; Reed et al. 1989). In reality the demands of training and competition of many athletes offers considerably less time, for recovery, in between sessions. Since the potential for performance in subsequent training sessions in part depends on the recovery of muscle glycogen storage, some athletes may compromise performance by initiating training with inadequate glycogen storages in the working muscles. To ensure an optimal outcome of the individual training session and thereby the overall performance development of the athlete, an efficient resynthesis of the muscle glycogen storages, after an exhaustive training session, is of great importance. This is in particular important in athletes with a high training volume and/or several training sessions a day. Based on these challenges regarding recovery in elite sports, several methods to increase muscle glycogen storages have been investigated. Research in this area has focused mainly on: timing, frequency, type and amount of carbohydrate ingestion as well as co-ingestion of other macronutrients, mainly protein (Carrithers et al. 2000). To optimally restore muscle glycogen storages athletes are recommended to consume high amounts (>1.2 g/kg/h) of high glycemic carbohydrates immediately after exercise and the following 4-6 hours. In order to ensure optimal glycogen storage as well as to avoid gastrointestinal distress the ingestion of post exercise meals should be consumed at frequent intervals (evenly at every 15 to 30 minutes) during these initial 4 to 6 hours following exercise (Burke et al. 2004).
Insulin is known to play an important role in the promotion of glycogen synthesis in muscles (Beelen et al 2010). In effect, the rate of carbohydrate uptake across the plasma membrane of the muscle cells seems to be controlled by the rate of insulin secretion from the pancreas (Jeppesen et al. 2000). Therefore increasing pancreatic insulin secretion and/or increase insulin sensitivity in the muscle tissue might be important for optimizing cross-membrane glucose transport and hence glycogen resynthesis after exhaustive exercise training.
Both exercise and the secretion of insulin have been shown to increase the rate of glycogen resynthesis (Chiang et al. 2009). It appears that the effect of exercise and insulin work through pathways, independent of each other, and they might both might work to facilitate glycogen resynthesis in an additive manner (Chiang et al. 2009). It could be speculated that if insulin secretion and/or insulin sensitivity could be enhanced in the post exercise recovery period, then glycogen resynthesis may be increased, causing a faster recovery of muscle glycogen storages post exercise.
Research has been directed to how various diet impact muscle metabolism and performance. In particular research has been focused on the consumption of the combination of carbohydrate and proteins after strenuous exercise to enhance muscle glycogen restoration.
Carrithers et al. have investigated the effects of postexercise eucaloric carbohydrate-protein feedings on muscle glycogen restoration after an exhaustive cycle ergometer exercise bout. In the study 7 male collegiate cyclists performed 3 trials each separated by one week. The diet investigated were 1) 100% α-D-glucose, 2) 70% carbohydrate—20% protein—10% fat and 3) 86% carbohydrate—14% amino acids. The results of this study suggest that muscle glycogen restoration does not appear to be enhanced with the addition of proteins or amino acids to an eucaloric carbohydrate feeding after exhaustive cycle exercise. In addition, the serum insulin and glucose responses among the three eucaloric feedings displayed no differences at any time throughout the 4-hour restoration period.
Insulin is also necessary for the uptake of amino acids to tissues and for protein synthesis. Proteins are the compounds comprised of amino acids and are the building blocks of tissue formation within the body. The synthesis of protein is the method by which muscles are constructed. The human body synthesizes protein from diet at a rapid rate while the body is growing through adolescence and into young adulthood. The rate at which protein is synthesized slows significantly after age 20. In fact between the age of 20 and 80 humans lose approximately 20-30% of their skeletal muscle mass.
This age-related loss of muscle mass is often referred to as “sarcopenia of old age” and is the consequence of complicated a multifactorial processes or disorders. A variety of intrinsic and extrinsic factors appear to be involved in the aging skeletal muscle. Changes in intrinsic factors associated with aging muscle include hormone, growth factor and systems associated with energy such as glucose or fatty acid metabolism, whereas intrinsic factors include diet, exercise, injuries and sedentary lifestyle.
Hence, there may be a correlation between the glucose metabolism and the synthesis of proteins so that an improvement of the rate of re-synthesis of muscle glycogen also may show a beneficial effect on the rate of protein synthesis in the muscles. In fact impairment of insulin action on muscle glycogen storage may play an important role in general on age-related changes in muscle mass. The muscle glycogen synthesis pathway is often found to be impaired with type 2 diabetes. Decreased insulin action with aging may be related to decrease in lean body mass and/or to the impaired ability of the muscle to respond to insulin (Carmeli et al.).
Fujita et al. investigated how aerobic exercise affects the anabolic response of skeletal muscle protein synthesis to insulin in healthy older subjects. The result of their research showed that a single bout of moderate aerobic exercise overcomes the muscle protein insulin resistance and restores the physiological anabolic response of muscle protein synthesis to insulin in older people. More specifically they showed that muscle protein synthesis significant increased during insulin infusion only if the infusion was preceded by a bout of aerobic exercise. The effect was directly associated with an increase in blood flow which in turn was accompanied by a significant increase in amino acid delivery and transport into muscle tissue.
Stevia rebaudiana Bertoni (SrB) is a shrub native to Brazil and Paraguay. The leaves from this plant contain a large amount of the steviol glycoside, stevioside, which is a non-caloric sweetener 300 times sweeter than sucrose. Extracts from Stevia rebaudiana have been used for many years in South America in the treatment of diabetes indicating that compounds in the extract may affect the glucose metabolism in a beneficial way.
Lailerd et al. have studied the effect of stevioside treatment on skeletal muscle glucose transport activity in both insulin-sensitive lean (Fa/−) and insulin-resistant obese (fa/fa) Zucker rats. In the study the rats were restricted to 4 g of chow two hours before start of the test. At the start of the test the rats were administered either 200 or 500 mg/kg body weight stevioside by gavage. Two hours later the rats were given a 1 g/kg body weight glucose load by gavage. Blood samples were then collected at 0, 15, 30, 60 and 120 minutes after glucose feeding. Their results showed that the acute oral administration of stevioside did not significantly affect fasting plasma glucose and insulin. Also the in vitro glucose transport activity in skeletal muscle was investigated. In this experiment one soleus and both epitrochlearis muscles were dissected and incubated with a stevioside-containing solution. The result of this experiments indicated that stevioside improves the insulin action on skeletal muscle glucose transport system in both insulin-sensitive lean and insulin-resistant obese Zucker rats in dose-dependent fashion. It was not possible, however, to determine whether the concentrations of stevioside that were effective in positively modulating in vitro glucose transport in insolated skeletal muscle can be achieved in vivo following oral administration of the compound.
Gregersen et al. (2004) have studied the acute effects of stevioside in type 2 diabetic patients. In the study the patients were given a standard meal supplemented with either 1 g of stevioside or 1 g of maize starch (control). The results of their study showed that stevioside suppresses the postprandial blood glucose level in type 2 diabetic subjects in average 18% and the circulating insulin levels tended to be increased by stevioside. The article mentions the author's earlier in vitro studies in isolated mouse islets, which showed a glucose-dependent insulin release to stevioside, whereas the insulinotropic effect of stevioside faded in the presence of normal to low glucose. It is therefore hypothesized that an elevated glucose level, as found in the diabetic state, is needed for stevioside to elicit its beneficial effects.
In another study Gregersen et al. (2006) investigated whether the combination of stevioside and soy bean protein isolate would show an improvement in the treatment of diabetes in Goto-Kakizaki rats. In the study adult male GK weighing 200-300 g at the age of 20 weeks were divided into four groups and fed for 4 weeks with different pellet diets: Group 1 received a standard carbohydrate-rich laboratory diet (chow), group 2 received chow+stevioside (0.03 g/kg body weight), group C received 80% soya bean protein+20% chow, and group 4 received 80% soya bean protein+stevioside (0.03 g/kg body weight). The results of this study revealed that the combination of stevioside and soy bean protein isolate has synergistic positive effects on the characteristic features of the metabolic syndrome, i.e. hyperglycemia, hypertension and dyslipidemia.
International patent application WO 2006/116814 describes a composition used to treat hyperglycaemia and associated conditions. In particular, the document discloses a composition comprising an extract from at least one plant from the genus Stevia and at least one bile salt. In the experimental part of the document adult Wistar rats are treated with stevioside, 20 mg/kg, orally, daily for 5 days. Both a group of non-diabetic rats and a group of diabetic rats were tested. 15 minutes after the 5th dose, the animals were exposed to an oral glucose tolerance test using 4 g/kg, orally. Glucose blood concentrations were measured before the treatment, after the 5th dose, before the glucose test and 30 minutes after the glucose test. It is concluded that treatment with stevioside alone (i.e. when not co-administered with the bile salt) conferred beneficial effects on glucose levels under increased loading during the glucose test. However, no significant decrease in glucose levels was seen before the glucose test (i.e. before administration of glucose) when compared to the control group. These results support the finding that stevioside shows a blood glucose decreasing effect only in cases where the blood glucose level is higher than the normal level.
International patent application WO 01/56959 is directed to a substance for use in a dietary supplementation or for preparation of a medicament for the treatment of non-insulin dependent diabetes mellitus, hypertension and/or the metabolic syndrome. Stevioside is mentioned as a preferred example of such a substance. In one of the experiments type II diabetic patients are given a standard meal supplemented with 1 g of stevioside orally. Blood samples were collected 4 hours later. The results show that stevioside reduced the post prandial blood glucose response by 18.5% compared to placebo and tended to stimulate insulin response in type II diabetic patients, even though the difference did not reach statistical significant level. The results further showed that stevioside significantly reduced the postprandial glycogen level and the postprandial glucagon like peptide-1 level.
In yet another study it has been shown that steviol glycosides inhibit 11β-hydroxysteroid dehydrogenase type 1 (Diabetes, Obesity and Metabolism. 10 (10): 939-49, 2008). 11β-hydroxysteroid dehydrogenase type 1 is the name of a family of enzymes that catalyze the conversion of inert 11 keto-products (cortisone) to active cortisol, or vice versa, thus regulating the access of glucocorticoids to steroid receptors. An increased production of cortisol may result in an increased insulin resistance.
It has not previously been reported whether the intake of stevioside, or any related compound such as steviol or a steviol glycoside, affect the muscle glycogen re-synthesis in human beings, whose muscles are depleted of glycogen due to for instance exhaustive exercise. Neither has it been reported whether the intake of stevioside, or any related compound such as steviol or a steviol glycoside, affect the muscle protein synthesis in human beings, whose muscles are depleted in protein mass due to lack of exercise.
The present inventors have surprisingly found a beneficial effect of stevioside intake on muscle glycogen depletion and loss of muscle mass. None of the published studies have provided any information indicating that stevioside or related compounds would beneficially affect healthy subjects. Such finding could indicate that another mechanism exists in addition to the known beneficial effect on insulin sensitivity seen only for diabetes type 2 subjects.